U.S. Pat. No. 7,564,706B discusses a power factor corrected (PFC) AC-DC converter which has a modified boost input and a modified buck output. The boost switch returns to the output, not to ground. A third switch connects to the input. This allows much of the input current to pass through the converter to the output. There is no input current measurement, but nearly ideal power factor correction is achieved through “natural modulation.” A preferred PFC AC-DC converter uses a variable DC-DC transformer on its output, as a post regulator, to provide dielectric isolation and to provide voltage level shifting. The output of the PFC AC-DC converter has the control characteristics of a buck converter, so it is a natural mate for the variable DC-DC transformer. The variable DC-DC transformer controls the output voltage for nominal and low input voltage. The AC-DC buck converter limits over-voltage transients.